1. Field of the Invention
This invention relates to plastic molding machinery and more particularly to an improved shut-off valve for a conventional injection molder and/or a structural foam injection molder, the valve providing communication between a plastic melt injection apparatus and a mold.
2. Description of the Prior Art
Originally only an open nozzle was used to provide communication between a plastic melt injection apparatus and a mold. However, the precise timing and quantity of plastic melt injected into the mold cannot be controlled with an open nozzle. Thus, valves were developed to control the timing and the plastic "shot size."
Prior art valves generally comprise a valve body with an internal, elongate passageway that defines a path for the flow of plastic melt. The passageway has a charging port at one end and a discharging port at the opposite end, and has a piston located therein for metering plastic through the discharge port. The piston is generally elongated and cylindrical, and is slidably mounted in a cylindrical housing that is disposed in the passageway so as to be parallel thereto. The housing is supported by a plurality of spider legs that extend radially from the housing and are rigidly attached to the walls of the valve body forming the outer boundary of the passageway. Generally, the legs are as thin as practicable so as not to unnecessarily restrict plastic melt flow, while being sufficiently sturdy to withstand the pressures developed in the valve.
However, this arrangement of spider legs nonetheless has several inherent and substantial disadvantages. Firstly, the legs divide the plastic melt flow into a plurality of individual flow paths. Thus for illustrative purposes only, if a mass of plastic melt progressed through a valve having four spider legs to secure the piston housing in place, the melt would be forced around these legs, thus dividing the plastic mass into four flow paths. Although the individual flow paths eventually merge into one flow path before the plastic melt is injected into the mold, the plastic, having "memory", retains to some extent these divisions which show up as weakened seams in the final molded product.
Secondly, as the plastic moves at a high velocity over the spider legs, frictional energy is generated which scorches or burns the plastic and the additives or chemical modifiers that has stagnated adjacent the legs. This burning causes melt degradation and poor rejoining of the sub-streams, resulting in yellowish or brownish streaks in the final product.
Thirdly, plastic tends to hand-up on the upstream portion of the spider legs, the valve requiring disassembly and cleaning on a regular basis so that the passageway does not become clogged. Additionally, this hung-up plastic, if it becomes dislodged and flows into the mold, produces irregularities in the final product.
Fourthly, the legs do restrict the flow of plastic melt, creating greater compressive forces that affect the structure of the foam cells. Also occurring immediately following the spider legs, are pockets of drastic expansion that also affect the foam cells by creating premature foaming.
A fifth disadvantage relates to the actual construction of the plurality of spider legs. The legs must be machined by an electro-discharged machining process and are thus relatively expensive.